1. Field of the Invention
The present invention relates to a workpiece levitating device for holding and conveying a workpiece in the form of a thin plate while keeping the workpiece airborne.
2. Description of the Related Art
For conveying a workpiece in the form of a thin plate such as a liquid crystal glass substrate for use in liquid crystal display units, a semiconductor wafer, or the like, there has been used a levitating device in the art for levitating and holding the workpiece with a fluid. After the workpiece has been levitated by the levitating device, the workpiece is moved while being kept out of contact with other objects.
One levitating device employs a porous body having a plurality of pores for levitating a workpiece in the form of a thin plate such as a liquid crystal glass substrate, a semiconductor wafer, or the like while the workpiece is being conveyed. Specifically, air is ejected from the pores in the porous body toward the workpiece to levitate the workpiece a certain distance in the air under the force of the ejected air. The workpiece which is levitated by the levitating device is conveyed to a desired position while being kept out of contact with other objects and also kept free of dust. For details, see Japanese Laid-Open Patent Publication No. 2000-62950, for example.
Such workpieces are conveyed in a clean room in order to prevent dust particles from being attached to the surface of the workpieces. In the clean room, a downward flow of air is produced from above workers working on the workpieces to prevent dust particles produced by the workers from being applied to the workpieces, which are typically glass substrates, semiconductor wafers, or the like.
With the levitating device disclosed in Japanese Laid-Open Patent Publication No. 2000-62950, depending on the shape and weight of workpieces to be levitated and held in the air, it is necessary to employ an increased number of porous bodies for ejecting air toward workpieces at an increased rate. As a result, the amount of consumed air that is ejected to levitate workpieces with the levitating device is increased, resulting in an increased running cost.
The increased amount of air ejected from the porous body toward the workpieces tends to disturb the downward air flow which is produced in the clean room. As a consequence, dust particles from the workers may possibly be applied to the workpieces.
Since the workpieces are generally in the form of thin plates, the levitating device is required not to strain or flex the workpieces when they are upwardly levitated and held in the air. The forces of the ejected air are applied to a workpiece only at regions above the respective porous bodies and nearby regions. Therefore, if the surface area of the workpiece levitated by the ejected air streams is large, then the porous bodies are spaced from each other by large distances, and the workpiece has a large area between the porous bodies to which the forces of the ejected air are not applied. As a result, those regions of the workpiece to which the forces of the ejected air are not applied tend to be strained or flexed due to gravity.